Some elements and attributes have been removed from the spec,
but are kept here for backwards compatibility.

Also, this makes gen.go support go:generate and write contents into
target files directly.

Finally, this makes `go generate` work on an entire directory.

Change-Id: I8d41840eec69eec1ef08527d8d71786612a3121d
Reviewed-on: https://go-review.googlesource.com/65152
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>